The present invention relates to a computer system, in which a plurality of server computers are mutually connected by communication lines.
In some computer systems, in each of which a plurality of computers are mutually connected by communication lines, a standby server computer is provided for recovering any one of the server computers broken down (see Japanese Laid-open Patent Publication No. 2007-133542).
In the normal state, the standby server computer monitors actions of the server computers. When an abnormal state of any one of the server computers is detected, the standby server computer takes over the action of the server computer in the abnormal state.
A virtual tape system, which is capable of controlling virtual tape unit at high speed by an interface for controlling magnetic tapes, now exists as an example of said computer system. The conventional virtual tape system will be explained.
A schematic view of a structure of the conventional virtual tape system is shown in FIG. 14.
The virtual tape system 10 comprises: a upper-level switch 11 capable of being connected to a main frame; a virtual tape unit 12; integrated channel processors (ICPs) 13 provided between the virtual tape unit 12 and the upper-level switch 11; and physical tape units 14. Integrated device processors (IDPs) 15 and virtual library processers (VLPs) 16 for managing the entire system are provided between the physical tape units 14 and the ICPs 13.
The ICPs 13 are connected to the IDPs 15 by an intermediate-level switch 17. The IDPs 15 are connected to the physical tape units 14 by a lower-level switch 18.
The ICPs 13 are server computes for reading data from and writing data in the virtual tape unit 12 and connected to terminal equipments of the main frame by the upper-level switch 11.
The VLPs 16 are server computers for controlling the entire system 10 and manages volumes of the virtual tape unit 12 and the physical tape units 14.
The IDPs 15 are server computers for backing up or restoring a logical volume of the virtual tape unit 12 in the physical tape units 14.
In the virtual tape system 10, a plurality of the server computers having the same functions are provided so as to form a redundant system.
Therefore, in the conventional virtual tape system 10, the ICPs 13, the VLPs 16 and the IDPs 15 are respectively connected in parallel, and the server computers having the same function monitor each other. In case that any one of the server computers having the same function comes into an abnormal state, the rest of the server computers supplement the action of the abnormal server computer.
In the above described conventional computer system, a plurality of the server computers having the same function are connected in parallel to form the redundant system, but the action of the abnormal server computer can be taken over by only the server computer having the function which is the same as that of the abnormal server computer. Therefore, in case that the abnormal state occurs, the system performs degenerate actions or essential actions only, so performance, reliability and redundancy of the system will be lowered.
Note that, in some conventional virtual tape systems, the server computers are accommodated in one housing. In this case, it is difficult to secure a space for installing a backup system for each of the server computers in the housing.